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Ship rats have no particular prejudice in favour of their own species, and can form close friendships even with much larger animals such as cats, dogs and rabbits provided these have been brought up with rats and know not to attack them. They often form really deep, emotional attachments to humans.
On the down side, they really like to tease their friends - and other animals don't always appreciate the joke, and may become hysterical when they find a mad rodent running up their legs and swinging on their tail if any. My steel doe Charcoal was never interested in shredding things until she saw the guinea-pig lay claim to a piece of kitchen-paper, and then she had to have it - waltzing round him darting in and out to snatch the paper from under him while he jumped up and down on the spot going "Wheeet wheeet!"
They particularly like to sit on top of other people's heads - human heads are a favourite, but feline or canine ones will do.
Be cautious about introducing ship rats to smaller animals. It is probable that, like Norway rats, ship rats would regard much smaller rodents such as mice and gerbils as prey - there are recorded instances of ship rats killing baby gulls, which would be about the same size as a large mouse. They would certainly not kill larger animals such as guinea-pigs, even if they wanted to, as their bite is comparatively weak.
All ship rats, but especially domi nant bucks, are likely to view members of their own gender as rivals to be fought and driven away - regardless of species. Very dominant bucks will even have a go at a dog, and will certainly attack male Norway rats, cats, guinea-pigs etc., pummelling them and urinating on them. My buck Murdoch especially liked to urinate on the cat's head and then trample up and down rubbing it well in with his feet.
It is highly unlikely that - as stated in many text-books - the aggressive Norway rat attacked and killed the gentle native Old English (ship) rat. Ship rats are by far the more quarrelsome species, and so much faster and more agile that it would be extremely difficult for a Norway rat to get to grips with a ship rat against its will. But having attacked a Norway buck - as they do - and got into a clinch with him, they may be in danger from the Norway rat's far more powerful bite: and it is possible that some Norway rats would attack a ship rat if they were in a confined space together.
If ship rats do get on well with another species they may become sexually attracted to it. In particular many ship does fancy Norway bucks, although the feeling isn't always mutual.
An additonal complication is that, whilst both Norway and ship does like to be chased, ship does are much faster than a Norway buck and cannot be caught by one unless they modify their behaviour. My steel ship doe Charcoal, unable to control her instincts to the point of not running away at all, used to take her elderly and slow-moving Norwegian boyfriend Campion down into the basement of their cage and then run round and round him in tight but furious circles until he reached out and grabbed her.
It is often but not always possible to keep ship and Norway rats together. Introductions should be carried out in the same way as same-species introductions - see section on social behaviour.
Nursing Norway does will feed orphaned ship kittens (instructions on how to introduce babies to a foster mother can be found in the section on reproduction), although this is not always successful as Norway rats tend to keep their nest at too low a temperature.
Baby ship and Norway rats can normally be introduced together and will grow up harmoniously. This occasionally fails due to aggression by the ship rat: but it is safe to try it, as there is no risk of a ship kitten seriously damaging a Norway kitten.
Adult females of mixed species, or baby and adult females, can usually get on OK: I have successfully kept such mixed groups. For a while I also had two ship does living loose in my flat, and while they would squabble with any Norway does who happened to be out, I have also seen one of these ship does pal up with a Norway doe she barely knew, and exchange polite social grooming.
I would be much more cautious about introducing males of mixed species and age: an adult Norway buck might seriously attack a baby ship buck and vice versa. Introducing adult males of different species would probably be impossible on both sides.
Some Norway bucks do not recognize ship rats as other rats - or not as potential sexual partners, anyway - and chase them off regardless of sex. Indeed there are accounts from the Victorian era of wild Norway rats actually killing and eating ship rats, apparently regarding them as big mice - but the situation was very abnormal, with both species locked together in a small cage without having been properly introduced; and the Victorians loved blood-curdling stories about how vicious Norway rats are and may well have exaggerated. I have never heard of a case of a domestic Norway rat killing a ship rat. I have seen a dominant domestic Norway buck take a female ship rat's throat in his mouth and look thoughtful, but he didn't actually bite.
If you do get a Norway buck who recognizes a ship doe as a female rat, they can live together very successfully and have a passionate sex-life without the risk of offspring. Ship buck and Norway doe is both more and less difficult: ship bucks usually do recognize Norway does as female and respond accordingly, but they have difficulty reading a Norway doe's heat-cycle, and may pester her for sex when she isn't in the mood.
Very dominant and/or antisocial ship bucks may actually attack a Norway doe who intrudes into their territory - though the doe will just get scrabbled and scratched and generally rolled around rather than seriously savaged.